When is the right time or date to stop replying to a thread?
When you have nothing else to add to the existing discussion,
And when the thread has been inactive for so long that it makes no sense to bump it up at that time and it offers now current benefit to users.
Why are some members not locking or closing their threads after they get answers off them? Only very few practice this on this forum.
They are either unaware of the lock option or the users just don't care enough to do it.
You can always report a thread you feel has run its course to the mods and they could consider locking it.